1. Field of the Invention
The present invention relates to a carcass structure for tyres for vehicle wheels, comprising at least a carcass ply having a first and a second series of strip-like segments consecutively arranged along the circumferential development of the carcass structure, each of said which extends according to a substantially “U” shaped conformation and comprises at least two filiform elements positioned longitudinally and parallel to each other and at least partially coated by at least a layer of raw elastomeric material, and a pair of annular reinforcing structures each engaged in proximity to a respective interior circumferential edge of the carcass ply.
2. Description of the Related Art
The manufacture of tyres for vehicle wheels entails the formation of a carcass structure essentially composed by one or more carcass plies shaped according to a substantially toroidal configuration and presenting their axially opposite lateral edges engaged to respective circumferentially inextensible annular reinforcing elements, usually called “bead rings”.
On the carcass structure is applied, in circumferentially exterior position, a belt structure comprising one or more belt strips shaped as a closed loop, essentially composed by textile or metallic cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies.
In a position circumferentially exterior to the belt structure, a tread band is then applied, normally constituted by a strip of elastomeric material of suitable thickness.
It should be specified that, for the purposes of the present description, the term “elastomeric material” means the rubber compound in its entirety, i.e. the set formed by at least a polymeric base suitably amalgamated with reinforcing fillers and/or process additives of various kinds.
Lastly, on the opposite sides of the tyre being manufactured, a pair of sidewalls is applied, each of which covers a lateral portion of the tyre lying between a so called shoulder area, located in proximity to the corresponding lateral edge of the tread band, and a so-called bead located in correspondence with the corresponding bead ring.
Traditional manufacturing methods essentially provide for the tyre components listed above to be first produced separately from each other, then assembled in a tyre manufacturing phase.
For instance, the manufacture of the carcass ply or plies to be associated to the bead rings to form the carcass structure requires first that, through an extrusion and/or calendering process, a rubberised fabric be produced comprising continuous textile or metallic cords, arranged longitudinally. This rubberised fabric is subjected to a transverse cutting operation to produce segments of predetermined dimensions, which are subsequently united to originate a continuous belt-like semi-finished product, having transversely positioned parallel cords.
This manufactured item must then be cut into segments of a length correlated to the circumferential development of the carcass to be produced.
Manufacturing methods have also been proposed which, instead of producing semi-finished products, produce the carcass structure directly during the tyre manufacturing phase.
For instance, U.S. Pat. No. 5,453,140 describes a method and an apparatus that form a carcass ply starting from a single cord previously wound on a reel.
According to the method and the apparatus described in this patent, at each operative cycle of the apparatus the cord drawn from the reel by means of motorised driving rollers and kept tensioned by means of a pneumatic tensioning system is cut to measure to obtain a segment of predefined length.
The cord segment is drawn by a gripping element mounted on a belt wound on motorised pulleys to be laid transversely onto the exterior surface of a toroidal support.
The ends of the segment are then engaged by belt folding organs operating on opposite sides of the toroidal support to apply the cord segment radially onto the toroidal support itself by means of cursor elements which act in the manner of fingers along the lateral portions of the segment
The repetition of the operative cycle described above leads to the laying of many cord segments in circumferential side by side relationship until the entire circumferential development of the toroidal support is involved.
Of necessity, the toroidal support is previously coated with a layer of raw rubber which has a dual function of adhering to the cords laid thereon in order adequately to hold them according to a fixed positioning, and of constituting an interior coating, impermeable to air, in the finished tyre.
Tyres obtained through this manufacturing method present a carcass structure wherein the carcass ply or plies are constituted by single cords each presenting two lateral portions axially distanced from each other and oriented radially to the axis of rotation of the tyre, and a crown portion extending in radially exterior position between the lateral portions.
Within the scope of the manufacture of the carcass structure, it is also known that in proximity to each of the beads of the tyre, the opposite ends of the single cords constituting a carcass ply are located, with alternating sequence, in axially opposite positions relative to an annular anchoring element constituting the aforesaid bead ring, shaped in the manner of an annulus composed by wire turns radially superposed on one another, as can be observed from the Patent EP 0 664 231 and from U.S. Pat. No. 5,702,548.
The cords that compose the carcass ply or plies, however, are positioned substantially according to the neutral axis of bending resistance of the respective bead. In this circumstance, the structural strength of the beads must necessarily be entrusted to the rigidity of filling inserts made of very hard elastomeric material incorporated in the structure of the bead, whose behaviour is affected by temperature variations due both to environmental factors, and to the stresses induced during normal operation.
In Patent FR 384 231, the production is proposed of a carcass structure by the laying, onto a toroidal support, of a series of rectangular bands made of rubberised fabric circumferentially positioned side by side one after the other and arranged according to radial planes relative to the geometric axis of the supporting drum itself. The laying of the bands is conducted in such a way that the terminal edges of two non-consecutive bands are partially covered by the terminal edges of the band interposed between them.
The spaces existing between the terminal edges covered by the bands are filled by means of trapezoidal inserts applied to the terminal edges of the band superposed thereon. The laying of the bands is effected according to several superposed layers, in a number correlated to the thickness to be conferred to the carcass structure. The presence of the aforesaid trapezoidal inserts determines a thickening of the carcass structure in the areas of the beads, providing it with double the thickness measurable in the rim.
U.S. Pat. No. 4,248,287 describes a method according to which the formation of the carcass structures provides for the laying, on a toroidal drum, of a plurality of layers each formed by radial strips composed by rubberised wires and set circumferentially side by side relative to each other. Once the laying is complete, two bead rings are applied in the bead area, around which bead rings are then folded back the terminal edges of the carcass layers formed by the radial strips.